In iCal 5.0, which is included with Lion, you can navigate between dates, weeks, months, or years using a finger swipe left or right using an Apple Magic Mouse or Trackpad.

Click on the Day, Week, Month, or Year button. Navigate between dates, weeks, months, or years using a finger swipe left or right. The left or right swipes will advance or reverse the displayed calendar.

With the Magic Mouse use a single finger swipe, and on a Trackpad use a two-finger swipe (you must check 'Swipe between pages' in their respective Control Panels).

[crarko adds: When I tried this with the Magic Trackpad, I could only get it to work reliably in the Weekly calendar view, so your mileage may vary.]

This doesn't work exactly as it says here for me. I would expect a three-finger swipe left to move forward the calendar like on an iPhone / iPad which would fit in with the rest of Lion's new scrolling. Unfortunately, it seems Apple has yet to fix this behaviour — a left swipe moves back for me and right goes forward. Can anyone else confirm this??

My Apple Trackpad (that is the external pad, this does not seem to work on a laptop trackpad) works with a two finger swipe. The number of fingers can be changed in the System Preferences>Trackpad>More Gestures>Sweep between pages. Interestingly for me is that when set to two-fingers the months change with the page flipping animation. When set to three fingers there is no animation between months.

The left-right behavior seems linked to the Scroll & Zoom>Scroll Direction option. When selecting "natural" the months go forward with a sweep to left (which feels "unnatural" to me). I have gotten used to the natural setting for up and down scrolling as it is more like the iPhone scroll. I have come to prefer sweeping for going through the months, weeks, or days over clicking, regardless of the direction selected.